1. Field of the Invention
This invention relates to methods of electroplating or electrodepositing a metal, particularly iron and iron alloys, onto a target substrate where the electroplating solution includes an additive, which increases the useful life of the solution. The additive is stable to air and reactive with Fe(III) species to form a Fe(III)-additive species. The electroplating solution is irradiated so that the Fe(III) in the Fe(III)-additive species is reduced to Fe(II), which is then available to be electroplated onto the target substrate. Additives, which appear to be particularly advantageous in the method, include, without limitation, hydroxycarboxylic acids and their lactones. Specifically, sugar acids, such as saccharic or glucaric acid, are especially preferred additives to the electroplating solutions of the present method.
2. Discussion of the Related Art
Electro less and electroplating processes are used to form a wide variety of coatings on various types of substrates. Of particular interest, is the formation of “soft” magnetic alloys on micro-magnetic devices such as magnetic recording heads and thin-film inductors and transformers. Using photolithographic photoresist techniques, desired patterns and shapes may be electroplated onto a target substrate.
An electroplating solution may include a source of metal cations for plating, such as an easily dissociated salt of the metal or metals to be plated, for example, the sulfate or the cyanide, and a buffer to maintain the desired solution pH. A complexing agent to lower the concentration of the free metal ion, and thus slow down the plating process may also be present in a typical electroplating solution. Retarding the plating process prevents a rough or brittle metal plating from forming on the plated substrate.
Electroplating may be utilized to plate films composed of complex alloys, such as CoFeX, NiFeX, and CoNiFeX alloys, where X includes such elements as P, B, S, or Mo. Typically, the X elements are added to the electroplating solution in the form of oxo-acids or salts thereof. See, for instance, U.S. Pat. No. 5,883,762.
The photochemistry of iron compounds, including Fe(III)-hydroxycarboxylic acid complexes, has been studied by various investigators. See, for example, Sima, et al., Coor. Chem. Rev. 160, pp. 161–189 (1997) and Kuma, et al. in Wat. Res., Vol. 29, No. 6, pp. 1559–1569 (1995). Kuma looked at, in particular, the photoreduction to Fe(II) by sunlight of ferric-hydroxycarboxylic acid complexes, including sugar acids, such as glucaric or saccharic acid, in seawater with a typical basic pH of greater than 7.
The use of reducing mono- and di-saccharides to reduce ferric iron in the electroplating bath to ferrous iron is described in U.S. Pat. No. 3,974,044, where in combination with complexing agents, such as hydroxycarboxylic acids, a high iron content plating alloy is obtained. The use of hydroxycarboxylic acids, such as citric acid, as complexing agents to keep both ferrous and ferric ions in solution, or as antioxidants, such as ascorbic acid, to keep ferrous iron in the ferrous state is also described in U.S. Pat. Nos. 3,806,429;3,795,591; and 3,354,059. However, none of those patents disclose irradiating the electroplating solutions to reduce ferric species to ferrous species.
For iron electroplating solutions, formation of ferric or Fe(III)-containing compounds is undesirable since they reduce the plating and current efficiency of the iron electroplating process. The reduction in current efficiency due to the build-up of Fe(III)-containing compounds severely decreases the time that a particular iron electroplating solution may be utilized to electroplate or electrodeposit material onto a target surface. Ferric ions also often precipitate out of the electroplating solution. The desired iron species for plating is ferrous or Fe(II) which is unstable in the presence of dissolved dioxygen (O2) and may be oxidized to ferric ions at the anode.
Some strategies that have been implemented to limit the concentration of Fe(III) in iron electroplating solutions include deoxygenation of the solution by various means, use of a soluble iron anode, and addition of a reducing agent, such as ascorbic acid. Reducing agent usually has to be continuously added to the solution to replace agent oxidized by both Fe(III) and/or oxygen. These oxidation processes also produce oxidation by-products, which may accumulate in the bath and lead to more deleterious effects.
Clearly, there is a need for electroplating methods which decrease the formation of ferric (FeIII) species, preferably to ppm levels or lower, and concurrently increase the concentration of ferrous Fe(II) species available for plating in an electrodeposition solution.